Delamination Morphology Maps of Chromium Oxide Scales Formed on Cr-Based Alloys
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RODUCTION
CHROMIUM- or aluminum-based alloys are commonly used for high-temperature applications. Their ability to form protective thermal oxides scale such as alumina or chromia reduces the kinetics of oxidation of these materials.[1–5] In particular, Ni-Cr and Fe-Cr alloys form a protective continuous scale of a-Cr2O3 at the surface of the metal in a middle-high range of temperatures (700 °C to 1000 °C). The durability of the resulting metal/oxide system will depend on the mechanical integrity of this oxide scale, but also on the strength of the alloy/oxide interface and therefore on damage occurrence during oxidation and cooling processes. These damages are closely correlated with the residual stress magnitudes and relaxation modes in the oxide and the alloy.[6,7] In particular, the buckling mechanics laws describe the eventual appearance of buckles in the scale,
M. GUERAIN is with the Unite´ Mate´riaux Et Transformations, UMR 8207, CNRS-Universite´ de Lille, Cite´ Scientifique, 59650, Villeneuved’Ascq, France. Contact email: [email protected] P. GOUDEAU is with the InstitutPprime, De´partement Physique et Me´canique des Mate´riaux, CNRS-Universite´ de Poitiers-ENSMA, SP2MI, Te´le´port 2, bd Marie et Pierre Curie, BP 30179, 86962, Chasseneuil Futuroscope Cedex, France. J.L. GROSSEAU-POUSSARD is with the Laboratoire des Sciences de L’inge´nieur pour l’Environnement UMR 7356, CNRS-Universite´ de La Rochelle Avenue Michel Cre´peau, 17042, La Rochelle Cedex 1, France. Manuscript submitted July 4, 2019. Article published online April 9, 2020 2822—VOLUME 51A, JUNE 2020
as well as their capacity to propagate or spall, which is damaging and thus not good for protecting the metallic structure. In order to better control such features, it is then mandatory to precisely determine the stress distribution in the oxide scale, to characterize the delamination, and finally to calculate the interface adhesion of the metal/oxide system. For this purpose, numerous works have already been done for chromia or alumina scales. For example, aluminum-based alloys have been investigated and correlations between high residual stress magnitudes and high percentage of scale surface damages have been evidenced.[8–11] Contrary to chromia scales, the damage features in alumina consist principally in spalling and it has been supposed that these spalls were formed from initial buckles. Indeed, no work could be done specifically on these buckles because they were difficult to evidence experimentally since the transition from buckles to spalls is spontaneous. For chromia forming alloys, damages consist in both buckles and spalls; these two types of features have been evidenced by several authors.[12–16] In particular, the buckles density was correlated with the evolution of the oxide scale growth stress.[16] A complete study of an oxide scale on metal system needs to take into account not only the residual stress magnitude in the scale and the number and the type of delamination, but also the interface toughness. Evans and Lobb calculated
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